Unto Him belongs all that is in the heavens and all that is on earth; and, verily, God - He alone - is self-sufficient, the One to whom all praise is due.
Al-Hajj (The Pilgrimage) Verse 64
Whenever the Prophet asked permission to enter, he knocked the door thrice with greeting and whenever he spoke a sentence (said a thing) he used to repeat it thrice.
Sahih Bukhari: Volume 1, Book 3, Number 94